The present invention relates to conveying electrical currents through high magnetic fields. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. The present invention will also find application in other fields in which high magnetic fields are present, particularly strong, pulsed magnetic fields.
Commonly, magnetic resonance imaging devices have a built-in gradient coil surrounding the patient receiving bore. For imaging smaller areas of the patient and conducting specialized studies, insertable gradient coils are sometimes inserted into the bore. These insertable coils include surface coils, head coils, biplanar gradient coils, and other gradient coils which can be received in the main field bore.
The insertable gradient coils are powered by current pulses to generate magnetic field gradients along selectable x, y, and z-axes. Flexible electrical conductors which extend through the bore from an external current source to the inserted coil carry these electrical pulses.
The cables which carry the current pulses to the insertable gradient coils for MRI imaging must conduct large currents. The cables must also be able to retract, i.e. coil and flex, within the bore to allow the insertable coil to be moved into and out of the magnet. When a current passes through a conducting cable in the presence of a strong external magnetic field, as is present in an MRI magnet bore, an orthogonal Lorenz force is generated. The Lorenz force, i.e. the force caused by the interaction of orthogonal components of current and magnetic fields, causes mechanical displacement of the cable. The forces can manifest themselves in the mechanical displacements of the cable or may give rise to torques which cause cable rotation.
Mechanical movement of the cable is disruptive and potentially dangerous to the patient. Further, the movements cause offensive acoustic noise which can be unnerving to the patient. Moreover, the movement and torques cause fatigue of the cable, particularly at the interconnection between the cable and the insertable gradient coil.
In the past, others have used paired, parallel conductors for insertable gradient coils. Such parallel or biaxial coils are particularly susceptible to mechanical movement. The two currents flow along displaced axes and through the same magnetic field. This causes opposite forces orthogonal to each of the displaced cable conductors causing torques and mechanical movement of the cable.
Others have suggested using twisted wire pairs. The twisted pairs of leads were twisted in a helix around the central axis and were mechanically constrained into the helical configuration. With the two wires of the twisted pair mechanically fixed together, the opposing forces balance and cancel, i.e. the net mechanical force is zero. However, the torque is not zero. Due to the non-zero torque, each current pulse to the gradient coils causes the cable pairs to twist, leading to vibration, mechanical noise, and potential connection and component fatigue.
Coaxial cable is used in many applications, primarily for high frequency, high voltage, low current transmission where characteristic impedance is important. The feed cables to insertable gradient coils must carry higher currents than normally carried by coaxial cables, but at lower voltages with less impedance characteristic criticality.
The present invention contemplates a new and improved cable lead assembly for insertable gradient coils which overcomes the above-referenced problems and others.